Undine (Paula Beer) works in Berlin as a historian and guide to the city's development. But underneath the appearance of her modern city life lurks an old myth: if the man Undine loves betrays her, she has to kill him and return to the water she once came from. So when her lover Johannes (Jacob Matschenz) leaves her for another woman, she thinks she has no choice - until in the moment of betrayal she meets Christoph (Franz Rogowski), an industrial diver, and unexpectedly falls for him.